My Chinese in-laws intend to visit us in one month from now. Their visa is valid from Nov 23rd.
Would it be possible for them to take the airplane on November 22nd in China and arrive in Schengen area on November 23rd?

Or does the visa already have to be valid upon departure?

In our particular case, they still need to transfer in Beijing first. The actual flight to Schengen zone effectively only departs on the 23rd so I guess that should be completely fine then?